The present invention relates to an optical waveguide soot preform having an improved compositional profile and to a method of making the same. More particularly, this invention relates to a preform having such a compositional profile that the preform aperture is readily closed to form a seed-free blank during the soot consolidation process or during the process of drawing a fiber directly from a soot preform. This invention also relates to the resultant optical waveguide fiber.
Certain glass making processes, particularly vapor deposition processes, have been commonly employed in the formation of optical waveguide blanks. In one such process, referred to as the flame hydrolysis process, a plurality of constituents in vapor form are entrained in a gaseous medium in predetermined amounts and thereafter are oxidized in a flame to form soot having a predetermined composition. The soot is applied to the surface of a rotating cylindrical mandrel or starting member. After a first coating of soot is deposited to form the core glass, the composition of the soot is changed to form a cladding glass coating. Heretofore, the mandrel has usually been removed and the soot preform has been sintered to form a consolidated glass draw blank that is free from particle boundaries. The resulting cylindrical, hollow draw blank is heated to a temperature at which the material has a low enough viscosity for drawing and is drawn to reduce the diameter thereof until the inner walls of the hollow member collapse. Continued drawing further reduces the diameter until an optical waveguide fiber having the desired dimensions is formed. Various methods employing the flame hydrolysis technique for forming glass optical waveguide fibers are taught in U.S. Pat. Nos. Re. 28,029; 3,711,262; 3,737,293; 3,823,995 and 3,826,560, the latter two patents being directed to the formation of gradient index waveguides.
The following problems have been encountered when practicing the method wherein the consolidated draw blank was formed with a longitudinal aperture caused by removal of the mandrel. Because of the dopants added to the core region of the blank to increase the refractive index thereof, the coefficient of expansion of that region is greater than that of the cladding region. Thus, when the blank cools after the consolidation process, the central region is put into a state of tension. The aperture in the center of the blank, which has remained after removal of the mandrel, presents a free surface in the region of high tensile force where breaks are readily initiated.
Prior to drawing the blank into a fiber it has been necessary to subject the blank aperture to an acid wash followed by careful rinsing to avoid contamination which could render the resultant fiber useless for the purposes for which it has been produced. This acid wash increased the cost of producing optical waveguides. Also, the rate of drawing the fiber has been limited by the necessity of closing the aperture of the draw blank during the drawing process.
The aforementioned disadvantages have been overcome by the methods disclosed in copending application Ser. No. 44,192 of M. G. Blankenship entitled "Method of Making Optical Devices" filed May 31, 1979, now U.S. Pat. No. 4,251,251, and in U.S. Pat. No. 4,157,906 issued to A. C. Bailey.
In accordance with the method disclosed in the Blankenship application the mandrel is removed, and the resultant hollow soot preform is subjected to a relatively high consolidation temperature. As the preform is heated the soot consolidates and, due to the predetermined relative viscosities of the core and cladding materials, the aperture closes. More specifically, the Blankenship application teaches that the aperture closing process is facilitated by selecting the core and cladding materials such that the ratio V.sub.1 /V.sub.2 of the viscosity of the core to that of the cladding is 1/2 or less, ratios as low as 1/2000 being operative.
The Blankenship application teaches that care must be taken to minimize damage done to the preform while removing the mandrel since such damage can result in the formation of seeds at the center of the resultant consolidated draw blank. Damage to the aperture can be reduced by forming a soft parting layer on the surface of the mandrel and thereafter building up the preform with hard soot. This parting layer can consist of soft soot, carbon or the like. A method of coating a mandrel with carbon is disclosed in U.S. Pat. No. 4,204,850 issued to E. K. Dominick et al.
Even when the aforementioned precautions have been taken, seeds can form along the centerline of the consolidated blank when the aperture is closed during consolidation. The tendency for such centerline seeds to form is dependent, inter alia, upon glass composition. Certain glass compositions capable of providing desirable optical properties could not be employed in the formation of soot preforms in which the aperture was to be closed during consolidation because of the tendency for centerline seed formation.
The method of the Bailey patent involves drawing an unconsolidated soot preform directly into a fiber. This method has also experienced centerline seed problems.